An obstacle detection device as described in patent literature 1 has been proposed for detecting existence of an obstacle around a vehicle or a distance from the vehicle to the obstacle by using an ultrasonic wave. The obstacle detection device described in the patent literature 1 can use two different modes, one being a short distance mode having a broad directivity and the other being a long distance mode having a narrow directivity, by changing transmission frequency and transmission output of an ultrasonic sensor. As such, an obstacle in a short distance from the vehicle and an obstacle in a long distance from the vehicle can be both properly detected.
In a clearance sonar system or a parking assist system in which the above obstacle detection device is employed, it is important for a proper vehicle control to accurately discriminate between the obstacle that needs to be avoided from contacting and a low bump that needs not to be avoided, thereby to restrict unnecessary detection. To avoid the unnecessary detection in which the low bump is detected as the obstacle, as described in the patent literature 1, it has been considered to perform the detection by narrowing the directivity of the ultrasonic sensor in a vertical direction.
However, even when the directivity of the ultrasonic sensor is narrowed, a reaching range of the ultrasonic wave spreads especially as the distance between the obstacle and the vehicle increases. Therefore, there is no difference between reception intensity of a reflected wave reflected by a wall and reception intensity of a reflected wave reflected by the bump. In such a case, there is a possibility that it cannot be determined precisely whether a reception result of the ultrasonic sensor indicates the wall or the bump. As a result, there is a possibility that the bump, which needs not to be avoided, is detected as the wall when trying to detect the wall.